feat(06-02): add sftp entry to registry, index, and rclone-conf

- Add sftp entry to BACKEND_REGISTRY with host, user, pass, key_pem fields
- Remove placeholder NOTE comment (sftp entry now present)
- Add sftp, onedrive, gcs, b2 to BACKEND_SCHEMAS in index.ts
- Add sftp, onedrive, gcs (google cloud storage), b2 to RCLONE_TYPE_MAP
- All 55 registry and rclone-conf tests pass; zero TypeScript errors

src/schemas/index.ts

@@ -21,5 +21,9 @@ export const BACKEND_SCHEMAS = {
   azureblob:        buildZodSchema('azureblob'),
   s3:               buildZodSchema('s3'),
   's3-compatible':  buildZodSchema('s3-compatible'),
+  onedrive:         buildZodSchema('onedrive'),
+  sftp:             buildZodSchema('sftp'),
+  gcs:              buildZodSchema('gcs'),
+  b2:               buildZodSchema('b2'),
 } as const;